Seed and seedling survival of the palm Astrocaryum murumuru and the legume tree Dipteryx micrantha in gaps in Amazonian forest

The role of regeneration strategies in structuring tropical tree communities has been a recent focus of attention of tropical ecologists. Canopy discontinuities created by tree-falls are considered to be critical to many rain forest trees for establishing and completing their life cycles. It was investigated whether it is advantageous for two common large-seeded and shade-tolerant Amazonian tree species, the palm Astrocaryum murumuru and the legume, Dipteryx micrantha, to have their seeds dispersed to gaps. The dispersal process was simulated by placing experimental seeds and seedlings in gaps and in the shaded understorey, and then following their performance. Seeds and seedlings from the two species were used. The Cox proportional hazard model was used to estimate the survival functions and compare survivorship curves. Survival of Astrocaryum seeds, and of Dipteryx seeds and seedlings, was higher in two gap zones (crown and bole) than in the shaded understorey (both near and far away from a conspecific adult). A higher proportion of seedlings of both plant species survived within gaps than in the shaded understorey over an 18-wk period. One year after the seedlings had been transplanted into gaps these patterns still held. Naturally occurring Astrocaryum seeds survived better beneath conspecific adult trees located within 30 m of gaps than beneath trees located 100 m from gaps. Variation in gap size was significant for Dipteryx seeds, but not for Dipteryx seedlings and Astrocaryum seeds and seedlings. Dipteryx seeds survived longer in smaller than in larger gaps. By colonizing tree-fall gaps, these two species improved their chances of completing their life cycles.